The present invention relates to a print apparatus and method for printing at a variable print speed.
Thermal printers are often used to print color images on work pieces such as plastic cards. A color image is created on a work piece by making multiple printing passes over the work piece with the printer printing a basic color, also referred to as a color canvas, on each pass so as to derive a composite color image once each color canvas has been printed. The three basic colors typically used are yellow, magenta, and cyan. Color data is sent to the printer for each pixel to be printed. This data might be sent in vary sizes, e.g., 4-bit, 8-bit, 16-bit, etc.
If 8-bit color data is used for each basic color, i.e., each pixel printed on the work piece has a shade value from 0-255 with 0 representing none of the basic color or zero optical density for a given pixel and 255 representing the maximum transfer of dye to the work piece or maximum optical density for a given pixel. It is often said that the data value of 0-255 represents the color shade or optical density of the color. Thus, if 8-bit color data is being used, there are 256 possible different shade values or optical densities for each basic color. By doing three different basic color passes so as to combine the three basic colors to create a composite color, a combination of more than sixteen million colors (256.sup.3) can be obtained for each pixel location on the work piece.
As noted above each basic color printed on the printer is referred to as a color canvas. Even though there might be 256 color shades available for each color canvas (as in the example of 8-bit color data), it is quite possible that the maximum shade value or optical density which is used or present in a given color canvas is less than the maximum possible data shade value. For example, the maximum shade value used in a given color canvas might be 100 whereas the maximum possible data shade value is 255 (where 8-bit color data is used).
Most thermal printers are limited in the number of color shades they can print. For example, a printer may only be able to print 128 different color shades even though 8-bit color data is being received for each color. Typically, a thermal printer has individual printer dot elements which are energized a varying number of times and/or length of time for each pixel of the color image to be printed depending on the shade value to be printed at that pixel. Typically this is done under control of a clock such that the printer dot elements are energized for the number of clock cycles necessary to print the shade value at each pixel. Most printers have an upper limit on the number of clock cycles per pixel or the number of times their printer dot elements can be energized per pixel which accordingly limits the number of color shades they can print.
Traditionally thermal transfer printing is done at a fixed speed as determined by either the media (receptor absorption rate) or the ribbon's dye transfer speed, and the rate at which data could be clocked out to the print head. Printers are designed to print at the worst case speed. Thus the printer must wait the entire time it would take to energize the printer dot elements to print all of the pixels on a color canvas as though they were at the maximum shade value. Although the receptor absorption rate and the dye transfer speeds define the absolute high end print speeds, there is substantial waste in efficiency by the printer having to print at the worst case speed.
The present invention solves these problems and other problems associated with existing printing apparatus and methods.